Visualization of acoustic wave fields has been the subject of extensive investigation with the result that a wide variety of demonstrated methods exist for converting a pattern of acoustic pressure fields in a fluid to a visible analog. A number of ultrasonographic methods have been devised that depend on the sensitivity to acoustic radiation of either photographic emulsions or certain chemical reactions.
However, sensitivities here are low (on the order of one watt per square centimeter) and exposure times of from minutes to hours are required. Numerous secondary effects of the acoustic energy absorbed by an insonified surface (e.g., luminescence, changes in color or electrical conductivity) have been used to produce both temporary and permanent images. Although these techniques are roughly an order to magnitude more sensitive than the previous ones, they are still too insensitive for practical diagnostic visualization. A somewhat more sensitive, but still slow to respond method is one in which the orientation produced by an ultrasonic field in a suspension of metal platelets is detected by optical scattering. The methods for acoustic imaging that have received the most widespread attention in recent years are those based on piezoelectric conversion of instantaneous acoustic pressures to proportional electric potentials. A two-dimensional ultrasonic pressure pattern in a fluid can be detected with great sensitivity by mechanically scanning a small piezoelectric probe over a region of the fluid through which the sound passes.
With each of the above methods, long exposure or scanning times preclude real time ultrasonic visualization. Real time visualization is, of course, highly important for medical applications. For example, the ability to observe an organ continuously as its aspect is varied, or (as may be possible with some organs) as the patient displaces it through muscular action, would be of considerable benefit to the diagnostician. To achieve this, a real time conversion method is required.
The method of optical Bragg diffraction (actually, not simply an image plane conversion method, since it embodies a distinctly different principle of image formation) is potentially capable of extremely high resolution. However, it is impractical for use at the low-megahertz frequencies required for diagnostics. The method of liquid surface relief is showing greatly improved image fidelity and sensitivity. However, with the present state of the art, the sensitivity is still marginal for diagnostic use and dynamic range sufficient for this application has not been demonstrated. Laser interferometer techniques of image conversion show promise, particularly because of their potentially high sensitivity and large image area. However, they have not yet achieved adequate sensitivity and the laser and other optical components require a large stable platform, which cannot be incorporated into a small camera unit. Another possibility, the Sokoloff tube, consists of a resonant quartz face plate on an electron-beam scanning tube. In spite of considerable effort to improve it, however, the Sokoloff tube still lacks adequate resolution and sensitivity, and is beset by reliability problems.
The best approach to real time visualization of acoustic wave fields appears to involve provision of an array or arrays of discrete piezoelectric receiving elements which are sequentially sampled in synchronization with a cathode-ray-tube display by electrical gate-circuits. Ideally, the entire image plane would be filled by a rectangular matrix of 40,000 to 100,000 receiving elements. However, the problems of producing and attaching an equal number of electronic switches and amplifiers to these elements in a confined space in a practical configuration and at reasonable cost are still beyond the state of the art. A good compromise is achieved, however, by using a hybrid converter consisting of a line array of discrete piezoelectric elements, electronically scanned at a high rate while the entire line is mechanically translated across the image plane or, alternatively, the acoustic image field is moved past the stationary array. A converter employing this general concept is described in an article by P. S. Green, J. L. S. Bellin and G. C. Knollman entitled "Acoustic Imaging in A Turbid Underwater Environment" which appears in the JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, Vol. 44, No. 6, December 1968, pp 1719-1730 (see pp 1726, 1727).